Diatoms are the most abundant phytoplankton in coastal waters. In temperate waters such as those of the Pacific Northwest, diatoms form large blooms each Spring and Fall, providing a seasonal input of organic carbon that fuels ecosystem dynamics. Diatoms are characterized by high levels of intra- and inter-specific genetic and physiological variation, perhaps explaining why they are so successful in a wide variety of environments. Under environmental conditions that remain largely unknown, the genus of pennate diatom, Pseudo-nitzschia, can produce the neurotoxin domoic acid. This compound can be biomagnified through the food web, ultimately culminating in a potentially lethal syndrome in humans known as amnesiac shellfish poisoning. Along the west coast of the U.S., the ecological and economic impact of domoic acid contamination of fisheries has increased dramatically over the past decade. In the Pacific Northwest, 4 species of Pseudo-nitzschia - P. australis, P. multiseries, P. pseudodelicatissima, and P. pungens -are associated with toxic blooms, which have led to closures of razor clam and Dungeness crab fisheries. All fishery closures to date in Washington State have occurred along the coast where human settlement is limited. In Puget Sound, where many more people could potentially be affected, no shellfish closures have yet occurred although low levels of domoic acid are commonly detected in Puget Sound mussels and clams. The goal of the research proposed in the Toxic Algae Research Core is to dissect apart the underlying reasons for the differences between the coastal environment of the Pacific Northwest and Puget Sound. We hypothesize that environmental variability in combination with genetic and physiological variation within and between the different Pseudo-nitzschia species results in variations in toxicity at the different sites. We will focus specifically on factors related to potential chemical/physical/genetic differences between the different environments and phytoplankton communities.